Vaccination to the current circulating influenza virus is routinely administered on an annual basis. While this vaccine is quite effective at protecting against the viral strains contained in the inoculum, it is less effective against variant strains. Whereas much information exists on the antibody response to influenza vaccination, the underlying cellular response of B cells that secrete these antibodies is only beginning to be understood. The human B cell response to influenza vaccination has two characteristic waves of specific cells that enter the blood: activated plasmablasts representing the ongoing immune response, and later the quiescent long-term memory B cells. This proposal investigates in situ the B cell differentiation events after vaccination that generates these plasmablasts and the subsequent memory response within lymphoid tissue following administration of the intranasal live attenuated influenza vaccine. Further, we will compare at the single cell level the relative cross reactivity, affinity, and neutralizing capacity of plasmablasts and memory cells generated in response to the monomeric 2009 H1N1 vaccine following immunization with the annual trivalent flu vaccine. This will provide important insight into the impact of a large and varied pre-existing memory repertoire on the nature of the human immune response upon rechallenge with evolving viral pathogens. Understanding better the immune response to current vaccine strategies will allow us to devise new strategies to generate more broadly neutralizing protection against influenza. PUBLIC HEALTH RELEVANCE: Annual influenza virus epidemics remain the cause of substantial illness among the general population and death in at risk populations. The challenge has been to design vaccines that induce long-term immunity against a pathogen that rapidly alters its appearance to the immune system by mutation and exchanging its components. By characterizing the mechanisms driving differentiation of both the short-term and long-term components of the humoral immune response, we can learn precisely what is required to make a protective antibody against influenza.